I slackened off the mounts to their minimal point without just dropping the motor and it seemed to make little difference.

It would seem odd that the motor would over heat, we haven't really ran the bot much, 10mins run time max, and none of those times were extended beyond a few minutes.

Maybe I'll swap both the motors out and see if that improves things, if that doesn't solve it then I guess having two extra motors isn't a bad thing. Process of elimination it is!